Field of the Disclosure
The present application relates to continuous wave (CW) lasers and inspection systems used to inspect, e.g., photomasks, reticles, and semiconductor wafers.
Related Art
As semiconductor devices' dimensions shrink, the size of the largest particle or pattern defect that can cause a device to fail also shrinks. Hence a need arises for detecting smaller particles and defects on patterned and unpatterned semiconductor wafers and reticles. The intensity of light scattered by particles smaller than the wavelength of that light generally scales as a high power of the dimensions of that particle (for example, the total scattered intensity of light from an isolated small spherical particle scales proportional to the sixth power of the diameter of the sphere and inversely proportional to the fourth power of the wavelength). Because of the increased intensity of the scattered light, shorter wavelengths will generally provide better sensitivity for detecting small particles and defects than longer wavelengths.
Since the intensity of light scattered from small particles and defects is generally very low, high illumination intensity is required to produce a signal that can be detected in a very short time. Average light source power levels of 0.3 W or more may be required. At these high average power levels, a high pulse repetition rate is desirable as the higher the repetition rate, the lower the energy per pulse and hence the lower the risk of damage to the system optics or the article being inspected. The illumination needs for inspection and metrology are generally best met by continuous wave (CW) light sources. A CW light source has a constant power level, which avoids the peak power damage issues and also allows for images or data to be acquired continuously.
Therefore, a need arises for a CW laser that generates radiation in deep ultraviolet (DUV) range, particularly shorter than 193 nm, and is suitable for use in inspection of photomasks, reticles, and/or wafers. However, at many wavelengths of interest, particularly ultraviolet (UV) wavelengths, CW light sources of sufficient radiance (power per unit area per unit solid angle) are not available, are expensive or are unreliable. If a beam source enabling CW output at near 183 nm at higher power level can be practically produced, it could enable more accurate and fast inspection/metrology and contribute to cutting-edge semiconductor production.
Pulsed lasers for generating Deep UV (DUV) light are known in the art. Prior-art excimer lasers for generating light at 193 nm are well known. Unfortunately, such lasers are not well suited to inspection applications because of their low laser pulse repetition rates and their use of toxic and corrosive gases in their lasing medium, which leads to high cost of ownership. A small number of solid state and fiber based lasers for generating light near 193 nm output are also known in the art. Exemplary lasers use two different fundamental wavelengths (e.g. US 2014/0111799 by Lei et al.) or the eighth harmonic of the fundamental (e.g. U.S. Pat. No. 7,623,557 by Tokuhisa et al.), either of which requires lasers or materials that are expensive or are not in high volume production. Another approach (U.S. Pat. No. 5,742,626 to Mead et al.) has not resulted in a commercial product with stable output and high power as required for semiconductor inspection applications (approximately 0.3 W or more is typically required in a laser that can run continuously for three or more months between service events). Moreover, most of these lasers have very low power output and are limited to laser pulse repetition rates of a few MHz or less. Recently, Chuang et al. has filed a patent (US Pub. App. No. 2016/0099540) on 183 nm mode-locked laser and related inspection system.
However, CW lasers with wavelength in the sub-200 nm are not commercially available at sufficient power level or very unreliable. An exemplary laser as described in U.S. Pat. No. 8,503,068 by Sakuma, may generate 193 nm CW radiation at about 100 mW with a complex apparatus comprising three fundamental lasers at different wavelengths, but the stability is really unknown. There have not been any prior-art for generating CW light in the wavelength range down to approximately 183 nm.
Currently available deep UV (DUV), i.e. a wavelength shorter than 300 nm, CW lasers operate by generating the fourth harmonic of an infra-red (IR) fundamental laser. Two frequency conversion stages are required. The first stage generates a second harmonic, and the second stage generates a fourth harmonic. Each frequency doubling stage uses a non-linear optical (NLO) crystal. The frequency doubling process depends on the square of the electric field strength. If the power density inside the crystal is low, the conversion process is very inefficient. An infra-red laser of a few Watts or a few tens of Watts of power, when focused into a non-linear crystal, produces very little second harmonic because of the low power density. This is in contrast to a pulsed laser of a similar average power level, which can produce substantial amounts of 2nd harmonic (in the best cases roughly 50% of the input can be converted to the second harmonic) because the peak power density is many times higher than the average power density.
DUV CW lasers use resonant cavities to increase the power density in the NLO crystals in order to improve the conversion efficiency. Most of the light that passes through the crystal without being converted to the second harmonic is recirculated in the resonant cavity so as to build up the power density. The second harmonic is allowed to pass out of the cavity. Eventually the power density builds up to a level where the power leaving the cavity as second harmonic plus the losses in the cavity equals the input power. In order to generate deep UV wavelengths two of these cavities must be connected in series. The first cavity generates the second harmonic (a visible wavelength, typically a green wavelength such as 532 nm) by recirculating the IR fundamental and the second cavity generates the fourth harmonic (a deep UV wavelength such as 266 nm) by recirculating the second harmonic.
FIG. 1 shows the major components of a prior-art deep-UV CW laser including two cavities. In this figure the cavity that generates the second harmonic comprises mirrors 110, 111, 112 and 113, and NLO crystal 115. The cavity that generates the fourth harmonic comprises mirrors 130, 131, 132 and 133, and NLO crystal 135. This figure also shows another important aspect of prior art devices. The resonant cavities need to be actively controlled. The control for the first cavity comprises oscillator 104 generating a signal at frequency f1, modulator 103, photodiode 105 and synchronous detector 106 which generates actuator control signal 107 to control the position of mirror 111. The control for the second cavity comprises oscillator 124 generating a signal at frequency f2, modulator 123, photodiode 125 and synchronous detector 126 which generates actuator control signal 127 to control the position of mirror 131.
IR light (at 1064 nm in wavelength) enters the first cavity through mirror 110 and, after reflecting from mirrors 111 and 112, enters NLO crystal 115. A portion of the IR light entering crystal 115 is converted to the second harmonic at a wavelength of 532 nm. The 532 nm light passes through mirror 113 and is directed to the second resonant cavity. Most of the IR light passing through crystal 115 emerges from the crystal without being converted and reflects from mirror 113, which is coated so as to reflect 1064 nm light while transmitting 532 nm light. Light reflected from mirror 113 arrives back at input mirror 110. The coating on mirror 110 is designed to be highly reflective to the IR arriving at the angle of incidence of the ray from mirror 113, while being highly transmissive to the incoming IR radiation arriving from the fundamental laser 101. In order to build up a high power density in the cavity, it is important that the IR radiation that has circulated around the cavity arrive at mirror 110 in phase with the incoming radiation. This is achieved as illustrated by a servo control which mechanically moves mirror 111 by means of a piezo-electric transducer or a voice coil to maintain the correct cavity length. Photodiode 105 monitors a small portion of the light circulating in the cavity in order to provide a signal to the servo control. The input laser beam is modulated by modulator 103 at frequency f1 in order to provide a time-varying signal that is used by the servo control to determine whether the cavity needs to be adjusted and in which direction the cavity should be adjusted.
The laser cavity servo control loop described above is commonly used and known as Pound-Drever-Hall or PDH control. Its theory is described by Dreyer et al. “Laser phase and frequency stabilization using an optical resonator”; Appl. Phys. B 31 (2): 97-105, (1983). Some additional details can be found in U.S. Pat. No. 5,367,531 and LIGO Technical note LIGO-T980045-00-D by Black (1998).
The other locking scheme commonly used in some laser servo control loop is called Hansch-Couillaud (HC) technique. In this locking scheme, no modulation is needed for the beam before entering the cavity, but it only works for cavities that are polarization sensitive. It detects the polarization change of the total reflected or transmitted beam to determine if the cavity is on resonance or not. Details can be found in the article by Hansch and Couillaud “Laser frequency stabilization by polarization spectroscopy of a reflecting reference cavity”, Opt. Commun. 35(3), 441 (1980).
The second cavity operates in a substantially similar manner to the first cavity except that the input wavelength is 532 nm and the output wavelength 266 nm. The coatings and materials of the second cavity components are chosen appropriately for those wavelengths. As shown in FIG. 1, a second modulator 123 modulates the light at frequency f2 prior to entering the second cavity. Photodiode 125 detects a small portion of the circulating light. The signal from 125 is used to generate a control signal 127 that controls the position of mirror 131 in order to maintain the correct length of the cavity.
In some prior art devices (not shown), the second modulator 123 is omitted and both servo loops operate at the same modulation frequency. In some prior art devices (not shown), neither modulator is present. Instead IR laser 101 generates a modulated output by operating the laser such that two modes are generated, those two modes being chosen to have a wavelength separation and relative amplitudes such that an appropriately modulated output is generated by the beating of the two modes.
In some prior art devices, the cavity may comprise two or three mirrors instead of four.
In some prior art devices, the DUV output wavelength may be separated from the recirculating visible light by a beam splitter (not shown) placed between the NLO crystal 135 and the cavity mirror 133.
The illumination needs for inspection and metrology are generally best met by continuous wave (CW) light sources. A CW light source has a constant power level, which allows for images or data to be acquired continuously.
A pulsed light source has an instantaneous peak power level much higher than the time-averaged power level of a CW light source. The very high peak power of the laser pulses results in damage to the optics and to the sample or wafer being measured, as most damage mechanisms are non-linear and depend more strongly on peak power rather than on average power. The higher the pulse repetition rate, the lower the instantaneous peak power per pulse for the same time-averaged power level. So in some cases, an additional pulse multiplier may be used to increase the repetition rate which adds more system complexity.
In addition, mode-locked laser typically have relatively broad bandwidth compared to CW laser. So the illumination optical system design in inspection/metrology tools is more complicated in order to minimize the aberration and increase the sensitivity, which also makes the system cost significantly higher.
Prior-art DUV CW lasers that generate the fourth harmonic of an infra-red (IR) fundamental laser could not produce wavelengths lower than 230 nm. At many lower wavelengths of interest, particularly ultraviolet (UV) wavelengths in the sub-200 nm range, CW light sources of sufficient radiance (power per unit area per unit solid angle) are not available, are expensive or are unreliable. There has not been any prior-art for generating CW light in the wavelength range down to approximately 183 nm.
Therefore, a need arises for providing an inspection system and associated laser systems that is capable of generating CW laser light having an output wavelength in the range of approximately 181 nm to approximately 185 nm and avoids some, or all, of the above problems and disadvantages.